Air separation

Abstract

Air is raised in pressure by a compressor and pre-purified by removal of impurities, particularly H 2 O vapor and CO 2 , in an adsorption unit. The resulting air is cooled in a heat exchanger, and is subjected to a first rectification in a rectification column in which the air is separated into a nitrogen-rich fraction and an oxygen-rich fraction. A further oxygen fraction enriched in argon is subjected to a second rectification in which argon is separated from oxygen in a second rectification column. First and second argon products of different purity are withdrawn from the second rectification column through outlets thereof.

We claim: 
     
       1. A method of separating air comprising: compressing, pre-purifying and cooling air;   subjecting said air to a first rectification in which the air is separated into a nitrogen-rich fraction and an oxygen-rich fraction;   withdrawing a further oxygen fraction enriched in argon from the first rectification; and   subjecting the further oxygen fraction to a second rectification in which argon is separated from oxygen and from which first and second argon product streams are withdrawn from said second rectification at different theoretical stages thereof so that said first and second argon product streams have different purities.   
     
     
       2. The method as claimed in claim 1, in which the first argon product stream contains less than about 10 volumes per million of oxygen. 
     
     
       3. The method as claimed in claim 1, in which the second argon product stream contains from about 1 to about 5% by volume of oxygen. 
     
     
       4. The method as claimed in claim 1, in which the second argon product stream is withdrawn continuously at a rate of from between about and about 50% of the total rate of argon product withdrawal. 
     
     
       5. The method as claimed in claim 1, in which the second argon product stream is withdrawn in liquid state. 
     
     
       6. An apparatus for separating air comprising: means for compressing, pre-purifying and cooling air;   at least one first rectification column for separating the air into a nitrogen-rich fraction and an oxygen-rich fraction;   said at least one rectification column having outlet means for discharging a further oxygen fraction enriched in argon;   said outlet means communicating with at least one second rectification column for separating argon from oxygen;   said at least one second rectification column having first and second product outlet means for withdrawing product argon streams;   said first and second product outlet means being arranged so as to enable first and second argon products having different argon concentrations from one another to be withdrawn.